This invention relates to methods for treatment of process chemicals and gases, and especially those methods which include reductive burning. It relates especially with respect to the reductive burning of chemical compounds containing certain anionic components, especially oxidized sulfur components, including sulfur-nitrogen components. Most especially, this invention pertains to the reductive burning of sulfate (SO.sub.4.sup..dbd.), thiosulfate (S.sub.2 O.sub.3.sup..dbd.), sulfite (SO.sub.3.sup..dbd.), and thiocyanate (SCN.sup.-).
These chemicals are commonly generated in certain manufacturing processes, such as flue gas scrubbing, the manufacture of TNT, the manufacture of resorcinol, the cleaning of coke oven gas, the pulping of wood for paper, and magnetohydrodynamic (MHD) electric power generation. There have been many attempts, particularly in connection with pulping processes and gas scrubbing processes, to find ways to chemically reduce these anionic components, in order that the process chemicals may be regenerated and reused. On the whole, certain of these processes have been partially successful, but they tend to leave a substantial portion of the anionic component in the unreduced state. These processes have been of such marginal success that it is common industrial practice in some places to dispose of the process chemicals, after certain treatments thereon, either as waste stream effluents to sewage treatment plants, for discharge into waterways, or for landfill. It would be highly desirable to be able to effect substantially complete reduction of these chemicals, with a minimum of polluting effluents, in a standardized process which would be effective for regeneration of starting materials in virtually any of the industries where such reduction is desirable.
The problem is particularly acute with regard to the recovery of chemicals from the scrubbing of hydrogen sulfide from industrial process gases. Of particular interest is the conversion of spent liquors or spent chemicals from scrubbing both hydrogen cyanide and hydrogen sulfide gases, from industrial process gases, to a treated liquor rich in sodium carbonate and sodium sulfide, with a minimum of sodium thiosulfate and sodium sulfate.
A process for recovering sulfur from scrubbing hydrogen sulfide from industrial process gases is well known as the Stretford Process, developed by W. C. Holmes of England. The process is described in various documents throughout the industrial literature. Illustrative of these descriptions is an article appearing in Chemical Engineering Progress magazine, October 1984, pages 40 through 47. Illustrative of another document describing the Stretford Process is the article by Carter, Rogers, and Morris, from the 1977 symposium from McMaster University in Hamilton, Ontario. Other descriptions of the Stretford Process are also believed to be known in the art.
The basic reaction of the Stretford Process includes the conversion of hydrosulfide (HS.sup.-) ions in an aqueous alkaline scrubbing solution to elemental sulfur through the use of oxidized vanadium catalyst. The vanadium is reduced in the process from a +5 valence to a +4 valence. The vanadium is regenerated to the +5 valence through the use of an anthraquinone regenerating composition.
In the operation of the Stretford Process, an undesirable side reaction takes place in that a small fraction of the hydrosulfide ions in the scrubbing solution is converted to the thiosulfate form. In the regeneration process, whereby the hydrosulfide is regenerated to sodium carbonate, the thiosulfate remains unaffected, and becomes an unreactive "dead load" in the Stretford Process stream.
The Stretford Process is known to have a maximum capability to carry sodium salt compounds, with a solubility capability of about 23% to 25% sodium salts under typical operating conditions. As the Stretford chemicals are reprocessed and recycled, the amount of non-reactive sodium thiosulfate in the solution gradually increases and the corresponding fractional amount (or concentration) of reactive sodium chemicals in the solution decreases. If the thiosulfate is not removed from the solution on a regular, or periodic, basis, the amount of sodium thiosulfate continues to build as the Stretford chemicals continue to be recycled through the reducing and oxidizing phases of removing elemental sulfur and regenerating sodium carbonate. Increasing circulation rates are then required of the Stretford solution in order to quantitatively provide enough reactive chemical in the scrubbing process to do the job. The limit of physical capability is reached when the concentration of reactive chemicals is so low that the circulating pumps cannot circulate the solution fast enough to provide the needed reactive chemicals.
As is indicated in the various literature articles written about the Stretford Process, substantial efforts have been expended toward the regeneration and recovery of Stretford solutions, and particularly with regard to the reduction of the thiosulfate component, so that the amount of unreactive sodium salts present in the solution does not excessively impede the scrubbing process. For example, U.S. Pat. No. 3,959,452 Espenscheid takes a portion of the spent Stretford liquor, commonly referred to as a purge stream, and treats it with either sulfuric acid or phosphoric acid followed by treatment with calcium hydroxide. While the result is an effective regeneration of the Stretford scrubbing chemicals, there is also included, as part of the result, the generation of substantial amounts of calcium sulfate or calcium phosphate which require landfill disposal. In the 1977 symposium at McMaster University, there was proposed a gas phase reductive burning process which claims a decomposition of approximately 73% of thiosulfate and 75% of sulfate, the sulfate having been produced as a side reaction to the reduction of thiosulfate. In 1979, Smith and Mills, in a chemical engineering symposium entitled Series Number 57, described a gas phase reduction reaction of Stretford chemicals. They indicate that it is "very difficult to decompose sulfate" and cite decomposition rates in the area of 60% to 70% of the sulfate. An April 1981 article in Chemical Engineering Progress also suggests reductive incineration of Stretford chemicals, with 60% to 70% of the sulfate being reduced to sulfide. Finally in European Application No. 0,072,278, by Dow Chemical Company in 1982, there is a suggestion that the Stretford chemicals should be recovered by the absorption of the anthraquinone component (ADA) and the vanadium component.
While these various processes indicate a degree of success in recovering spent reaction chemicals, there remains a substantial portion, particularly of the sulfate, which remains in the oxidized state after the chemicals have been through the recovery process.
It is an object of this invention to provide a method for the recovery of spent Stretford chemicals by a process which includes reductive burning, whereby the method will convert the anionic components, particularly of sulfate, thiosulfate, and thiocyanate, to sulfide and carbonate. It is another object to provide a process which is compatible, in general, with reductive burning of various anionic components, including sulfate, thiosulfate, and thiocyanate, and other anionic components. As part of the invention, it is an object to provide certain compositions of matter which are useful in their own rights and in the disclosed processes.